A novel quasi-one-dimensional topological insulator in bismuth iodide β-Bi4I4

Abstract

Recent progress in the field of topological states of matter has largely been initiated by the discovery of bismuth and antimony chalcogenide bulk topological insulators (TIs; refs ,,,), followed by closely related ternary compounds and predictions of several weak TIs (refs ,,). However, both the conceptual richness of Z2 classification of TIs as well as their structural and compositional diversity are far from being fully exploited. Here, a new Z2 topological insulator is theoretically predicted and experimentally confirmed in the β-phase of quasi-one-dimensional bismuth iodide Bi4I4. The electronic structure of β-Bi4I4, characterized by Z2 invariants (1;110), is in proximity of both the weak TI phase (0;001) and the trivial insulator phase (0;000). Our angle-resolved photoemission spectroscopy measurements performed on the (001) surface reveal a highly anisotropic band-crossing feature located at the point of the surface Brillouin zone and showing no dispersion with the photon energy, thus being fully consistent with the theoretical prediction.

Unusual Dirac Fermions on the Surface of a Noncentrosymmetric α -BiPd Superconductor

“…This observation is in good agreement with a theoretical prediction on HgS, which suggests that such an asymmetric Dirac state exists when there is a broken fourfold rotation symmetry [20]. In addition, there exist very few experimental reports discussing the asymmetric Dirac cone [22][23][24]. The states reported in Ref.…”

“…23 for the case of Sr/CaMnBi 2 cannot be classified into one of the 1D, 2D or 3D type of Dirac cone because, it is in the arc shape rather than cone shape and moreover the Fermi velocity varies between Γ − M and Γ − M . The Dirac states in Ru 2 Sn 3 are of quasi-1D [22], while the states in β-Bi 4 I 4 are of 1D but dispersing in the out-ofplane [24] in contrast to the in-plane dispersing asymmetric Dirac states of α-BiPd. Thus, the anisotropic Dirac states of this compound are different compared to the existing asymmetric Dirac states.…”

“…Therefore, the Dirac states found in this compound are of one dimensional (1D) character which could provide a natural route for the quantum wires. In general, the Dirac states found on the surface of 2D compounds such as graphene [14] [24]. Unusual band structure of this material, surface state is gapped and asymmetric, make this a unique compound which will attract tremendous future research interests.…”

Pressure effect and superconductivity in theβ−Bi4I4topological insulator

“…These 1D building blocks are held together by weak non-covalent bonds, forming 2D layers parallel to each other that in turn pile up making a 3D bulk single crystal [5,7]. Both first-principles electronic structure calculations and angle-resolved photoemission spectroscopy (ARPES) studies demonstrate that β-Bi4I4 is at the boundary of a strongweak topological insulator phases and a trivial insulator one [4]. Pressure or chemical doping could drive the material in one or another phase inducing a topological phase transition [4,8] as already observed in β-As2Te3 [9] and PbTe [10].…”

“…Both first-principles electronic structure calculations and angle-resolved photoemission spectroscopy (ARPES) studies demonstrate that β-Bi4I4 is at the boundary of a strongweak topological insulator phases and a trivial insulator one [4]. Pressure or chemical doping could drive the material in one or another phase inducing a topological phase transition [4,8] as already observed in β-As2Te3 [9] and PbTe [10]. Moreover, pressure can efficiently modify the small bandgap and the band structure of TIs, enhancing the thermoelectric properties, without introducing additional disorder like chemical doping would do [11].…”